X-ray backscatter systems can be used to perform non-destructive testing (NDT) for a variety of materials and applications. One important benefit of an X-ray backscatter system is that it requires access to only one side of the object under test. In a typical X-ray backscatter system, an X-ray generator is operated continuously at a fixed frequency and the resulting X-rays are highly collimated to form a beam that is directed on the target. As this beam is scanned over the target, X-rays penetrate the target and interact with the materials of the target and scatter in various directions. A detector suitable for detecting the scattered X-rays is located near the X-ray generator and is used to detect the backscattered X-rays and produce a signal therefrom. The X-ray backscatter system records the backscattered signal for each position of the X-ray beam on the target during a scan and thereby forms a pixel for each position. The resulting scan of the target is a collection of pixels that contain the X-ray backscattered intensity for each pixel. This collection of pixels can be displayed as an image. Typical images provide for viewing objects, defects, and structures buried within the target.
The usefulness of X-ray backscatter systems could be improved by the addition of depth information for each pixel. If the depth of the interactions were collected, or a distribution of depths for each pixel were available, it would provide a signal-to-noise ratio enhancement of the image by allowing only the signals from a selected depth to be viewed. This would also provide depth information for the defects or objects found within the target that could aid in remediation or further analysis.
Laser radar (LIDAR) systems that implement frequency modulation have been used to measure distances to targets located far from the LIDAR system. Since light photons generally reflect from the outer surface of the target, such systems cannot provide information regarding the depth of features within the target. Although light radiation and X-ray radiation both comprise photons, the X-ray and gamma-ray portions of the electromagnetic spectrum provide unique challenges because X-ray and gamma-ray photons cannot be easily focused and split using optical elements as in most LIDAR systems. Attempts to determine the depth of an interaction using conventional X-ray backscatter systems have involved large, costly and complex collimators, arrays of detectors, and coded apertures.
What is needed, therefore, is a method for determining the depth of interactions within a target using an X-ray backscatter system that does not require the high cost and hardware complexity of multiple detectors, collimation, and coded apertures.